The Stovebolt Site Forums The Parking Lot Big Bolts Vacuum 2 speed shift brush up!

Hello, Need a little brush up on the mechanics of the vacuum 2 speed shift on my 46 2 ton. It does not want to shift out of low range into high range at all almost like no vacuum going to the rear. Does not the control on the dash close one port and open another allow it to actuate the shift in the rear? I checked the lines going to the rear and they seem ok ,but don't want to start tearing into things until someone can refresh my memory. Thanks for any input, Jim

Before you do any troubleshooting make sure you chock the truck, it can roll free if you get the actuator shifted between ranges...

There should be two lines at the rear actuator, one for Hi range and one for Lo. Mark one of the lines with a wrap of tape so you know which port it went in case you mix them up.

Then remove the lines and see if you have vacuum at one of them (put your thumb over the end of the tube). If you have vacuum in one , move the switch and see if the other line now has vacuum. If both have vacuum then the issue is with the Actuator (bad diaphragm), if only one has vacuum when moving the switch than the issue is with the switch. if you have no vacuum at either line than the issue is between the switch and the engine...start looking for a disconnected or broken line.

If you find vacuum at only one line regardless of the switch position you can still test the actuator by switching the vacuum lines which should move it to the other range. If there is no change then you may have multiple issues...

Curious, hows the engine running?

Mike B

Thanks Mike, The engine runs great ,and holds at 25 on the vacuum gauge like it always did. Took the line off manifold connection ,and have good vacuum. That's why I wanted a brush up, been awhile since messing with one of these, never had too many problems with vacuum, electric was another story! Thanks for the reply, Ill let you know what I find, Jim

I Found that WD-40 works wonders on the vacuum valve on the dash. I replaced all the lines with 3/8 brake line and vacuum hose on the ends. I couldn't get the valve to work. Finally I just let the vacuum pull the WD-40 in. The valve stopped leaking and it worked great. First I would take the pancake plate off the side of the diff and pull the shift rod back and forth to make sure it's free while spining the wheels

Thanks Chris, I don't think I have a leak, but cant be positive, Ill give it a try!

Also for all you guys that 2 speed diffs. There's is a really cool reproduction sticker out there for the 46 era from MONGOS GARAGE vintage GM truck parts ($10.) Shows the low-high positions of the switch. (mine isn't marked on the truck). Show's Instructions how to shift and speed ranges for shifting.